1. Field of the Invention
The present invention relates to an offset rotary press, and more particularly to a gear train arrangement for driving at least a pair of printing cylinders or rolls such as a plate cylinder or roll, a blanket cylinder or roll and the like in an offset rotary press.
2. Description of the Prior Art
The offset printing has it as a basic operation principle to effect printing by transferring patterns on a plate surface on to a blanket surface, and thereafter transferring the patterns transferred onto the blanket surface onto a surface of a paper.
In a heretofore known offset rotary press, for example, as shown in FIG. 1, a first plate cylinder or roll 1 having a plate mounted thereon, a first blanket cylinder or roll 2 having a blanket mounted thereon, a second blanket cylinder or roll 3 and a second plate cylinder or roll 4 are disposed succesively adjacent to each other. A paper 5 is fed between the first and the second blanket cylinders 2 and 3. The first plate cylinder 1, the first blanket cylinder 2, the second blanket cylinder 3 and the second plate cylinder 4 are respectively driven in such a manner that adjacent ones of these printing cylinder may be rotated in the opposite directions to each other at the same revolution speed, to effect printing.
With regard to a gear train arrangement for driving the above-mentioned respective printing cylinders, for example, as shown in FIG. 2, it has been known that helical gears 10, 11, 12 and 13 having the same pitch circle diameter and the same number of teeth are fixedly mounted as by keys not shown to cylinder shafts 6, 7, 8 and 9, respectively, of the respective printing cylinders so that the gears may mesh with each other, and that a bevel gear 14 is fixedly secured to one of the cylinder shafts, e.g., the cylinder shaft 6 in FIG. 2, to this bevel gear 14 is meshed another bevel gear 16 which is fixedly secured to a vertical drive shaft 15 connected to a drive source not shown, and thereby driving power is transmitted to the respective printing cylinders so that adjacent ones of the printing cylinders may rotate at the same revolution speed in the opposite directions to each other.
In other words, with regard to a gear train arrangement for driving printing cylinders in an offset rotary press, there is known such a gear train arrangement that power transmission is effected successively from an upstream side to a downstream side with respect to a drive source according to a sequence of array of printing cylinders in which a first printing cylinder firstly transmitted with power from the side of the drive source, that is, the first plate cylinder 1 in FIG. 2 is disposed on the most upstream side of the sequence of array of printing cylinders and subsequently the downstream sides are given to other printing cylinders in the sequence of adjacent disposals.
It is to be noted that while driving power could be transmitted from the side of the gear 13 by fixedly securing the bevel gear 14 to the cylinder shaft 9 of the second plate cylinder 4, it is not favourable and hence not practiced commonly to fixedly secure the bevel gear 14 to the cylinder shaft 7 or 8 of the first or second blanket cylinder 2 or 3, because the cylinder shafts 7 and 8 are moved upon cylinder trip ON/OFF when the contact between the respective printing cylinders is touched/released.
As described above, in an offset rotary press, a plate cylinder and an associated blanket are driven through gears so that they may be rotated at a revolution speed ratio of 1:1. Since gears are generally provided with a backlash so that their meshed rotation can be achieved smoothly, among the gears rotating in a meshed condition, if a gear on the driven side is subjected to any external force, then the driven gear can move freely by the amount corresponding to the above-mentioned backlash with respect to the gear on the driving side.
On the other hand, in an offset rotary press, it has been well known, for instance, in an equal diameter cylinder arrangement or in a true-rolling cylinder arrangement, to make a finished diameter of a plate cylinder, that is, a diameter including a printing plate which has been mounted on the plate cylinder and a finished diameter of a blanket cylinder, that is, a diameter involving a blanket which has been mounted on the blanket cylinder to be different from each other, though slightly.
Thus, when the plate cylinder and the blanket cylinder having slightly different finished cylinder diameters rotate in an associated rotating condition as a result of a contact pressure for transfer of patterns, the rotational angles of the plate cylinder and the blanket cylinder would differ from each other by a small amount because the circumferential lengths of the respective cylinders are different, and since these cylinders are rotated while forcibly correcting this difference by gear drive, a larger force would act on the gears.
Accordingly, in FIG. 2, in the case where the finished cylinder diameter of the first and second blanket cylinders 2 and 3 are slightly larger than those of the first and second plate cylinders 1 and 4, respectively, then since the first blanket cylinder 2 which rotates in an associated rotating condition with the first plate cylinder 1 tends to rotate more slowly than the first plate cylinder 1, a tooth surface 10a on the leading side in the direction of rotation of the gear 10 comes into press contact with a tooth surface 11b on the trailing side in the direction of rotation of the gear 11 as shown in FIG. 3, whereas since the second plate cylinder 4, which is rotated in an associated rotating condition with the second blanket cylinder 3, tends to rotate faster than the second blanket cylinder 3, a tooth surface 13a on the leading side in the direction of rotation of the gear 13 comes into press contact with a tooth surface 12b on the trailing side in the direction of rotation of the gear 12 as shown in FIG. 4, thereby the rotation of the second plate cylinder 4 is constrained by the gear 13 and the gear 12 to forcibly correct the difference in a rotational angle between the printing cylinders 3 and 4, and the second plate cylinder 4 would rotate as advancing by the amount corresponding to the backlash S of the gear 13.
In other words, in the case where among adjacent printing cylinders, a printing cylinder having a larger finished cylinder diameter is positioned on the upstream side of the drive source with respect to another printing cylinder having a smaller finished cylinder diameter the other printing cylinder would rotate as advancing by the amount corresponding to the backlash of the gear.
However, rotation of a printing cylinder in an associated rotating condition caused by a contact pressure between printing cylinders lacks definiteness, because the rotation may be possibly varied even by momentary change of the contact pressure, and so a printing cylinder having a smaller finished cylinder diameter, that is, the second plate cylinder 4 in FIG. 2 would rotate as arbitrarily changing its position in the range corresponding to the backlash wherein forcibly correction by the gear is ineffective.
More particularly, if a torque causing associated rotation of the second plate cylinder 4 becomes small as a result of variation of the contact pressure, then the situation is changed in such a manner that a tooth surface 12a on the leading side in the direction of rotation of the gear 12 in FIG. 2 which is a driving gear and a tooth surface 13b on the trailing side in the direction of rotation of the gear 13 in FIG. 2 which is a driven gear would come into press contact with each other, and hence the second plate cylinder 4 is rotated by the driving gear, and therefore the second plate cylinder 4 can rotate freely by the amount corresponding to the backlash of the gear 13.
From the above-mentioned reasons, the contact positions of the printing cylinders, which rotate in contact with each other, necessarily lose definiteness, hence upon transfer of patterns from the plate surface to the blanket surface, deviation would arise in the circumferential direction of the blanket cylinder between previously transferred patterns remaining on the blanket surface and newly transferred patterns, and accordingly there was a problem that upon transfer from the blanket surface to a surface of a paper, the two previous and new patterns deviating from each other on the blanket surface would be printed in the form of doubled patterns.